Ink-jet printing has become an established printing technique and generally involves the controlled delivery of ink drops from an ink containment structure, or reservoir, to a printing surface.
One type of ink-jet printing, known as drop-on-demand printing, employs a pen that has a print head that is responsive to control signals for ejecting drops of ink from the ink reservoir. Drop-on-demand ink-jet pens typically use one of two mechanisms for ejecting drops: thermal bubble or piezoelectric pressure wave. The print head of a thermal bubble type pen includes a thin-film resistor that is heated to cause sudden vaporization of a small portion of the ink. The rapid expansion of the ink vapor forces a small amount of ink through a print head orifice.
Piezoelectric pressure wave pens use a piezoelectric element that is responsive to a control signal for abruptly compressing a volume of ink in the print head to thereby produce a pressure wave that forces the ink drops through the orifice.
Although conventional drop-on-demand print heads are effective for ejecting or "pumping" ink drops from a pen reservoir, they do not include any mechanism for preventing ink from permeating through the print head when the print head is inactive. Accordingly, drop-on-demand techniques require that the fluid in the ink reservoir must be stored in a manner that provides a slight underpressure within the reservoir to prevent ink leakage from the pen whenever the print head is inactive. As used herein, the term underpressure means that the fluid pressure within the reservoir is less than the pressure of the ambient air surrounding the reservoir. The units of underpressure measurement are given in positive values of water column height.
The underpressure in the reservoir must be strong enough for preventing ink leakage through the print head. The underpressure, however, must not be so strong that the print head is unable to overcome the underpressure to eject ink drops. Moreover, the ink-jet pen must be designed to operate despite environmental changes that cause fluctuations in the underpressure.
A severe environmental change affecting reservoir underpressure occurs during air transport of the pen. In this instance, the ambient air pressure drops as the aircraft gains altitude. This ambient air pressure drop reduces the underpressure level within the pen reservoir. If the underpressure reduction is not regulated, the underpressure will diminish to a level that is too low to keep ink from leaking through the print head.
The underpressure of an ink-jet pen reservoir is also subjected to what may be termed "operational effects." A significant operational effect on the reservoir underpressure occurs as the print head is activated to eject drops. The consequent depletion of ink from the reservoir increases the reservoir underpressure level. Without regulation of such underpressure increases, the ink-jet pen will eventually fail because the print head will be unable to overcome the increased underpressure to eject ink.
Past efforts to regulate ink-jet reservoir underpressure in response to environmental changes and operational effects have included various mechanisms that may be collectively referred to as accumulators. Examples of accumulators are described in U.S. patent application Ser. No. 07/289,876, entitled METHOD AND APPARATUS FOR EXTENDING THE ENVIRONMENTAL RANGE OF AN INK JET PEN CARTRIDGE.
Generally, prior accumulators comprise an elastomeric bladder or cup-like mechanism that defines a volume that is in fluid communication with the ink-jet pen reservoir volume. An accumulator is designed to move relative to the reservoir in response to changes in the level of the underpressure within the reservoir. Accumulator movement changes the overall volume of the reservoir to accommodate the underpressure level changes. As a result, the underpressure within the reservoir remains within an operating range that is suitable for preventing ink leakage but permits the print head to continue ejecting ink drops.
For example, as the underpressure within the pen decreases as a result of ambient air pressure drop, the accumulator moves to increase the reservoir volume to prevent the underpressure in the reservoir from diminishing to a level outside the operating range discussed above. Put another way, the increased volume attributable to accumulator movement prevents the underpressure drop that would otherwise occur if the reservoir were constrained to a fixed volume as ambient air pressure dropped.
Accumulators also move to decrease the reservoir volume whenever environmental changes or operational effects (for example, ink depletion during operation of the pen) cause an increase in the underpressure. The decreased volume attributable to accumulator movement keeps the underpressure from rising to a level outside of the operating range, thereby permitting the print head to continue ejecting ink.
Accumulators are usually equipped with resilient mechanisms that continuously urge the accumulators toward a position for increasing the air volume in the reservoir. The effect of the resilient mechanisms is to retain a sufficient minimum underpressure within the reservoir (to prevent ink leakage) even as the accumulator moves to increase or decrease the reservoir volume.
The effectiveness of an accumulator can be measured by the magnitude of the reservoir volumetric increase or decrease (that is, the magnitude of the pressure compensation range) that is provided for a given size of accumulator. Moreover, it is desirable that the accumulator consume as little space as possible so that the presence of the accumulator does not substantially reduce the ink capacity of the pen reservoir.